Biphenols, including substituted biphenols are known compounds, and are used as chemical intermediates and as monomers. 3,3',5,5'-Tetramethyl-2,2'-biphenol (TMBP) is similarly useful, see for example U.S. patent application Ser. No. 121,105, filed Jul. 23, 1998. Biphenols can be made by the oxidative coupling of (mono)phenols, but often many different products are obtained, other types of products such as ketones are obtained, and/or overall yields are poor for other reasons. This reaction is particularly useful where the phenol is substituted with electron donating groups.
Various oxidants may be used for the oxidative coupling of phenols, and one useful combination of reagents is a transition metal and an oxidizing agent, such as persulfate anion, see for instance R. G. R. Bacon et al., J. Chem. Soc., p. 2275-2280 (1954); S. L. Cosgrove et al., J. Chem. Soc., p. 1726-1730 (1951); C. G. Haynes et al., J. Chem. Soc., p. 2823-2831 (1956); and A. G. M. Barrett et al., Tetrahedron Letters, vol. 34, p. 2233-2234 (1993). The oxidation of 2,4-dimethylphenol (DMP) to TMBP has also been carried out by similar procedures and for the most part the yields of the desired TMBP have been low to moderate. It is noted that some of the oxidants and/or cocatalysts involve the use of relatively expensive metal compounds, which is a disadvantage for large scale commercial use.
R. G. R. Bacon et al., J. Chem. Soc., p. 1339-1344 (1960) report the preparation of TMBP from DMP in relatively low yield using a combination of persulfate anion and silver cation. No mention is made of using iron in place of the more expensive silver.